1. Field of the Invention
The present invention relates to a coating method and a coated product, particularly to a coating method and a coated product which have applications for coating a web with thin uniform layers at a high speed.
2. Description of the Related Art
Necessities for coating a web with thin uniform layers at a high speed have been arisen in order to follow improved functions and qualities of recent magnetic recording medium, photosensitive material, electronic material, coated battery, optical film for preventing reflection, polishing tape, data recording paper, and so forth.
To present a specific example, in manufacturing of a magnetic record medium, magnetic layers have been developed to be thinner and multiplied in order to follow larger scales of media. Specially, needs for making magnetic recording layers have been dramatically increased due to spread of a magnetic head such as an MR head which has a high sensitivity, and thus a coating method is required for forming an extremely thin film of a magnetic recording layer with thickness of between 0.02 xcexcm and 0.2 xcexcm in a dry state (between 0.2 xcexcm and 2 xcexcm in a wet state). Since the surface of the magnetic layer of this type requires an extremely high flatness, a conventional coating method attempts to improve capability of the magnetic recording layer by providing a non-magnetic coating layer with thickness of between 0.2 xcexcm and 3 xcexcm as a bottom layer of the magnetic recording layer (a layer next to web) so that projections over the surface of the web are flattened. In view of the above points in terms of productivity in manufacturing the magnetic recording layer, it is important whether or not the extremely thin magnetic recording layer and other layers can be formed on the web uniformly at a high speed.
Methods and apparatuses for forming the magnetic recording layer with thickness of 0.5 xcexcm or less at the dry state have been suggested in prior arts such as Japanese Patent No. 2581975 and Japanese Patent Application Laid-open No. 6-296917. Those coating methods and apparatuses use a method for applying and forming two coating layers at the same time in which the air is prevented from being mixed with the layers by applying a non-magnetic bottom layer in a suitable amount; hence the methods and apparatuses can form the magnetic recording layer of the top layers with thickness of 2 xcexcm in the wet state.
Further, the coating methods and apparatuses disclosed in the prior arts such as Japanese Patent No. 2942938 suggests a coating method in which the coating liquid for the bottom layer in low viscosity is applied on the web beforehand, and the coating liquid for the magnetic recording layer is applied by preventing mixture of the air with the layers while removing a part of the coating liquid for the bottom layer.
Yet a problem remains in which streaks with an equal pitch occur on the surface of the magnetic recording layer when the thickness of the coating layer for the magnetic recording layer is made to be 0.2 xcexcm or less (2 xcexcm or less at the wet state), even though the above-mentioned conventional coating methods and apparatuses are used. The streaks cause lowering capability of the magnetic recording layer as a product.
The fact is not limited to application of the magnetic recording layer. In application for layers other than for manufacturing the magnetic recording layer, the streaks with the equal pitch occur when applying multiple layers of two or more layers by applying an upper layer before the coating liquid for a lower layer is dried, so-called a wet-on-wet manner, especially when the thickness of layers other than the bottom layer is made to be 2 xcexcm or less.
In view of the above, a coating method is desired by which the thickness of the multiple coating layers is 2 xcexcm or less, or preferably 1 xcexcm or less at the wet state but the streaks with the equal pitch still do not occur.
The present invention has been developed in view of the above-described circumstances, and has as its object the provision of a coating method and a coated product which can prevent the streaks from occurring on the surface of the coating layers even though the coating layers, particularly the layers that are formed by multiple layer coating with the thickness of 2.0 xcexcm or less, or preferably 1.0 xcexcm or less at the wet state, are formed by a high speed coating.
In order to achieve the above-described objects, the present invention is directed to a coating method in which a web which continuously runs is relatively pressed against an edge face of a coating head and a plurality of coating liquids are discharged from a plurality of slits formed in the edge face in a direction of width of the web so that layers of the coating liquids are formed on the web, wherein: a capillary number Ca which is calculated by an equation Ca=Uxcexc/"sgr" satisfies an inequality 0.1 less than Ca less than 4, where U (m/s) is a running speed of the web, xcexc (Paxc2x7s) is an average high shear viscosity of the coating liquids, and "sgr" (N/m) is a surface tension of the coating liquid for a top layer of the layers.
In order to achieve the above-described objects, the present invention is also directed to a coating method in which a coating liquid is oversupplied to a web which continuously runs to form a bottom layer and then the web is relatively pressed against an edge face of a coating head and one or more of coating liquids are discharged from one or more of slits formed in the edge face in a direction of width of the web while an upper stream side of the edge face removes a part of the bottom layer when the coating liquid of the bottom layer is wet so that one or more of coating layers are formed on the bottom layer, wherein: a capillary number Ca which is calculated by an equation Ca=Uxcexc/"sgr" satisfies an inequality 0.1 less than Ca less than 4, where U (m/s) is a running speed of the web, xcexc (Paxc2x7s) is an average high shear viscosity of the one or more of coating liquids, and "sgr" (N/m) is a surface tension of the coating liquid for a top layer of the layers.
Moreover, in order to achieve the above-described objects, the present invention is also directed to coated products produced in accordance with the above-described methods.
According to the present invention, the coated product manufactured by the coating method can have a thin coating layer with the thickness of 2.0 xcexcm or less in the wet state and the coating layer does not have the streaks of the equal pitch, resulting in that capability of the magnetic recording layer, which is the coated product, can be dramatically improved if the coating layer is the magnetic recording layer, for example.
Moreover, in order to achieve the above-described objects, the present invention is also directed to a coating method in which a web which continuously runs is relatively pressed against an edge face of a coating head and a plurality of shear thinning coating liquids are discharged from a plurality of slits formed in the edge face in a direction of width of the web so that layers of the plurality of coating liquids are applied on the web in a wet-on-wet condition, wherein: viscosities of the plurality of coating liquids are adjusted so that a following inequality is satisfied,
xe2x88x9215xc3x9710xe2x88x923 less than xcexcn-1xe2x88x92xcexcn less than 5xc3x9710xe2x88x923, 
where xcexcn (Paxc2x7s) is a high shear viscosity of the coating liquid forming an n-th (nxe2x89xa72) layer from the web and xcexcn-1 (Paxc2x7s) is a high shear viscosity of the coating liquid forming an (nxe2x88x921)-th layer that is adjacent to the n-th layer.
The present invention is also directed to a coating method in which a shear thinning coating liquid is oversupplied to a web which continuously runs to form a bottom layer and then the web is relatively pressed against an edge face of a coating head and one or more of shear thinning coating liquids are discharged from one or more slits formed in the edge face in a direction of width of the web while an upper stream side of the edge face removes a part of the bottom layer when the coating liquid of the bottom layer is wet so that one or more of coating layers are formed on the bottom layer in a wet-on-wet condition, wherein: viscosities of the coating liquids are adjusted so that a following inequality is satisfied,
xe2x88x9215xc3x9710xe2x88x923 less than xcexcn-1xe2x88x92xcexcn less than 5xc3x9710xe2x88x923, 
where xcexcn (Paxc2x7s) is a high shear viscosity of the coating liquid forming an n-th (nxe2x89xa72) layer from the web and xcexcn-1 (Paxc2x7s) is a high shear viscosity of the coating liquid forming an (nxe2x88x921)-th layer that is adjacent to the n-th layer.
The present invention is also directed to coated products produced in accordance with the above-described methods.
According to the present invention, the coated product manufactured by the coating method can have a thin coating layer with the thickness of 1 xcexcm or less in the wet state and the coating layer does not have the streaks of the equal pitch, resulting in that capability of the magnetic recording layer, which is the coated product, can be dramatically improved if the coating layer is the magnetic recording layer, for example.